1. Field of the Invention
The present invention pertains to a liquid crystal display device that uses a cold cathode fluorescent lamp as a light source and in particular relates to the surface of a fluorescent substance film that is formed on an inner wall surface of the cold cathode fluorescent lamp.
2. Description of the Related Art
In image display apparatus that use a liquid crystal display panel that is a non-emitting type, an electronic latent image that has been formed on the liquid crystal display panel is visualized by disposing external illuminating means. In the external illuminating means, an illuminating device is installed on the back surface or the front surface of the liquid crystal display panel with the exception of a structure that utilizes natural light. Particularly in a display device that requires high luminance, a structure where the illuminating device is disposed on the back surface of the liquid crystal display panel is becoming mainstream. This is called a backlight.
Broadly speaking, there are side edge type backlights and direct type backlights. Side edge type backlights have a structure where a linear light source represented by a cold cathode fluorescent lamp is installed along a side edge portion of a light guide plate that comprises a transparent plate, and side edge type backlights are heavily used in display devices that must be made thin for personal computers and the like. On the other hand, in large-size liquid crystal display devices such as display devices that are used in display monitors and television receivers, direct type backlights are heavily used. Direct type backlights have a structure where the illuminating device is installed directly under the back surface side of the liquid crystal display panel.
A typical cold cathode fluorescent lamp that is used in this type of illuminating device is configured such that a pair of cathodes are installed in both end portions of a transparent glass tube, a fluorescent substance film is adhered to and formed on the inner peripheral surface of the glass tube, and mercury and a noble gas are enclosed inside that glass tube. Additionally, a high voltage is applied between the pair of opposing cathodes that are hermetically sealed in both end portions inside of this glass tube to cause the cathodes to discharge, whereby ultra violet rays with a wavelength of about 254 nm resulting from excitation radiation of the mercury are generated, and the fluorescent substance is excited by those ultra violet rays and visible light is radiated, whereby a luminous flux is obtained.
It is known that, usually the lumen maintenance factor of a cold cathode fluorescent lamp gradually drops because of lighting over a long period of time, and this tendency becomes even larger by increasing the electric current that flows in the cold cathode fluorescent lamp in order to improve luminance. However, from the standpoints in recent years of improving the screen luminance and lowering the costs of liquid crystal displays, increasing the luminous flux radiated from one cold cathode fluorescent lamp is much needed.
As a cold cathode fluorescent lamp that solves this problem, in JP-A-2006-24548, there is disclosed a cold cathode fluorescent lamp where the cross-sectional shape of a discharge space in a glass tube is set to a long diameter and a short diameter such that the cross-sectional shape of the discharge space is defined in a slender flat shape or an elliptical shape, whereby a planar portion is disposed on an installation side of the cold cathode fluorescent lamp, so that the cold cathode fluorescent lamp can be easily fixed and it is ensured that the installation direction can be unified, and where a light discharge surface resulting from the elliptical portion is disposed so as to face a light incident surface side of a backlight device, whereby light diffuses and luminance non-uniformity can be reduced, and the luminance can be raised.
Further, in JP-A-2003-323865, there is disclosed a cold cathode fluorescent lamp where the inner surface of a glass tube is covered by a protective layer comprising a metal oxide, this protective layer is covered by a fluorescent substance layer whose film thickness is less than 20 μm, and the glass tube is formed by a glass material that includes an excitation component which, when ultra violet light resulting from excitation is received by its light emitting substance, excitation-radiates ultra violet light whose wavelength is longer than that of this ultra violet light, whereby the utilization efficiency with which ultra violet rays resulting from excitation of mercury are utilized for the luminous flux is improved.